But Thursday, the administration signaled it could back the idea under certain conditions.

In a letter to Republican Senator John McCain of Arizona and Democrat Fritz Hollings of South Carolina, Transportation Security Agency director James Loy called for pilots to have lockboxes to store their weapons so they are not left in the cockpit.

Mr. Loy said only pilots who volunteer to carry weapons and receive extensive training should be armed. He also warned the plan could be expensive $900 million to launch it and $250 million each year thereafter.

Senator McCain said there are other concerns raised by the plan.

"There are some difficult issues that need to be addressed, among them are training, cockpit modifications, coordination among other nations and international airlines; for example landing in a country that has stricter gun control laws," he said.

Senator Boxer said the legislation was purposely written to leave such matters for the Transportation Security Agency to decide.